For questions about the plugin, open a topic in the Discuss forums. For bugs or feature requests, open an issue in Github.
For the list of Elastic supported plugins, please consult the Elastic Support Matrix.

If the event has field "somefield" == "hello" this filter, on success,
would add field foo_hello if it is present, with the
value above and the %{host} piece replaced with that value from the
event. The second example would also add a hardcoded field.

Add a unique ID to the plugin configuration. If no ID is specified, Logstash will generate one.
It is strongly recommended to set this ID in your configuration. This is particularly useful
when you have two or more plugins of the same type, for example, if you have 2 grok filters.
Adding a named ID in this case will help in monitoring Logstash when using the monitoring APIs.

XML in the value of the source field will be expanded into a
datastructure in the target field.
Note: if the target field already exists, it will be overridden.
Required if store_xml is true (which is the default).

xpath will additionally select string values (non-strings will be
converted to strings with Ruby’s to_s function) from parsed XML
(using each source field defined using the method above) and place
those values in the destination fields. Configuration:

xpath => [ "xpath-syntax", "destination-field" ]

Values returned by XPath parsing from xpath-syntax will be put in the
destination field. Multiple values returned will be pushed onto the
destination field as an array. As such, multiple matches across
multiple source fields will produce duplicate entries in the field.